A multi-input, multi-output (MIMO) link refers to a communication system with multiple antennas at a transmitter and a receiver. MIMO links are a known, effective way to increase link capacity, in terms of bits per second per hertz, of wireless links without increasing the bandwidth and power budget. It has been shown that with n_T transmit antennas on a transmit side and n_R antennas on a receive side, the capacity gain is proportional to n=min [n_T,n_R]. The capacity gain results from the formation of MIMO channels and n spatial channels.
There may be at least two benefits from multiple antenna systems; spatial diversity and beamforming. Spatial diversity refers to the ability to collect independent received samples containing the same piece of transmitted information. By combining different independent observations, the effect of channel fading can be reduced. Beamforming is used to steer the antenna pattern so that a strong beam is oriented toward the desired signal and weak beams or even nulls, are formed for interference signals. It has been shown that when complete feedback of the channel state matrix is available to the transmitter, the optimal transmission scheme for a MIMO link is a combined adaptive power control and eigen-beamforming scheme, based on instantaneous channel fading.
However, complete feedback is not easy to achieve especially when the number of transmit and receive antennas is large.